The present application hereby claims priority under 35 U.S.C. xc2xa7119 on German patent application number 10207288.4 filed Feb. 21, 2002, the entire contents of which are hereby incorporated herein by reference.
The present invention generally relates to a method for drilling holes in a substrate by way of a laser beam, in particular in an electric circuit substrate. The laser beam may be centered, with its axis on the substrate, in a region of the hole to be drilled and then moved within a cross-sectional area of the hole to be drilled with a beam spot diameter that is smaller than the diameter of the hole to be drilled in concentric circular tracks with a radius changing in steps.
A drilling method is disclosed in U.S. Pat. No. 5,593,606, where holes with a larger diameter than a beam diameter of a laser being are produced by virtue of the fact that the laser beam is moved from inside to outside or from outside to inside either in spiral tracks or in concentric circles within the region of the hole. Moving the laser beam in a spiral track results in asymmetry of the drilled hole, in particular the end of the spiral track leaving behind a step in the course of the rim of the hole.
It is expedient when drilling by way of circular movements of the laser beam always to cover full circles, in order to produce a hole that is as circular as possible. In this case, the beam is usually moved on the shortest path, that is to say in the radial direction, in each case when changing from one circular track to another of different radius. The starting points for all circular tracks consequently all lie on a radial line. When the laser outputs further pulses during this radial movement between the individual circular tracks, this has the effect of an additional input of energy in the direction of this radial line and, overall, of an asymmetric energy distribution in the hole area. However, if the laser is switched off during the radial movement, the result nevertheless is an unclean drilling course in the transition region between the radial movement and the circular movement, because in each case after the laser is switched on again the first pulses have a somewhat different energy profile than the remaining pulses. These asymmetries are also all situated in a radial direction and therefore add together to form an asymmetry at the rim of the hole.
It is also disadvantageous with this type of beam guidance that the laser beam has to accomplish a change in direction of 90xc2x0 in each case upon transition from the radial movement to the circular movement, and must be stopped briefly in each case for this purpose and started up in the new direction. The overall result is an unfavorable movement cycle for the deflecting unit of the laser.
An embodiment of the present invention may improve a method for laser drilling in such a way that the movement cycle of the laser beam in the hole region is more favorably configured, and that the shape of the hole can properly shaped, that is formed in a proper circular shape.
An embodiment of the present invention provides a method where the transition from one circular track to the respective next circular track having a different radius takes place in each case in the form of an arc which departs approximately tangentially from the circular track last traversed and nestles approximately tangentially against the circular track newly to be described, in such a way that in each case the starting point of a new circular track is offset by a prescribed angle from the starting and end points of the preceding circular track.
The offset of the starting points on the individual concentric circular tracks that asymmetries in the energy input that occur in each case at the start of a circular track, for example owing to inadequate suppression of the initial pulses, are distributed over the entire circumference, thus producing holes that are as symmetrical as possible, that is to say circular. Moreover, substantially no sharp changes in direction of the laser beam are required owing to a direct transition from a radial movement to a circular movement, as a result of which it is possible overall to achieve a more uniform speed of the individual elements in the deflecting unit and a more uniform pulse train, and this means in turn that the energy distribution on the individual circular tracks and in the entire hole to be drilled is rendered more uniform.
The starting points of the circular tracks traversed by the laser beam during drilling of a hole are preferably distributed uniformly over the entire circumference. Depending on the material of the hole to be drilled, the hole size and the laser properties used, the individual circular tracks can be traversed once or else several times. This can happen in a way that each circular track is traversed several times one after the other before the laser beam is then directed onto the next circular track. However, it is expedient to traverse all the circular tracks once each with offset starting points in a cycle in each case, and once again to traverse each circular track once each in a next cycle, the starting points of the individual circular tracks differing in each cycle from the preceding cycle.
The shape of the arc in the case of the offset movement from one circular track to the next can be selected to differ. Particularly advantageous in this case is the shape of a quarter ellipse, an offset between the starting and end points of approximately 90xc2x0 being produced in each case.
It is expedient for the laser beam to be controlled such that an energy input occurs in each case only when the laser beam moves on one of the circular tracks, while no energy input occurs on the arc between the circular tracks. This can be done, for example, by switching off the laser beam in the time of the arcuate movement between two circular tracks. Another, particularly advantageous possibility is that the laser beam, which outputs pulses of high energy density during the productive drilling movement on the circular tracks, is switched over into a continuous wave mode of low energy density in each case on the arcuate sections between the circular tracks.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.